    \begin{tikzpicture}
    %------------------------------------------------------------------------------------
    %    图片2.5
        \node at (0,0) {Z}
        [level distance=4em, sibling distance=4em]
            child {node {x} edge from parent[dash phase=0pt,dash pattern=on 1pt off 1pt on 1pt off 1pt on 1pt off 1pt on 10pt]}
            child {node {A} edge from parent[dash phase=0pt,dash pattern=on 1pt off 1pt on 1pt off 1pt on 10pt]
                child {node {$\alpha$} [dash phase=0pt,dash pattern=on 1pt off 1pt on 1pt off 1pt on 10pt]}
            }
            child {node {y} [dash phase=0pt,dash pattern=on 1pt off 1pt on 1pt off 1pt on 1pt off 1pt on 10pt]};
    \end{tikzpicture}